Under many circumstances it is necessary to pull in or pay out a cable or the like but it is impossible to wind this element around a drum. Instead the element must pass under tension straight through the device. Accordingly a device is known which has a normally horizontal lever pivotal about a horizontal transverse axis on a fixed support and having two attachments flanking the pivot. Arms pivoted on the attachments each carry a pair of jaws between which the cable passes, and which automatically lock on and grip the cable when pulled up along it in one direction, but which slide in the other direction. Thus as the lever is rocked back and forth about its axis the cable is first gripped and lifted by one of the grippers while the other slides down it, and then is clenched by the other gripper as it lifts while the one gripper slides down the cable to start lifting again. With a long lever arm the system can exert enormous forces, and can be operated by a crank or hydraulic cylinder for nonmanual operation.
The problem with such an arrangement is that it advances the cable and its load in steps. In addition at any given time only one of the sets of gripper jaws is engaging the cable, so that it must grip it with enormous force, frequently biting into and damaging the cable. The disadvantage of this concentrated gripping force at one location is compounded by the start-and-stop advance. In addition this jerky advance frequently damages the load itself, and subjects the cable to peak loads well in excess of the actual mass being lifted.